Paul is standing at the front of Ms. Lawrence's fourth-grade class. He looks small and bashful in jeans and a denim shirt, yet at the same time earnest and determined as he delivers a message to a visitor to his Bronx classroom at PS 86. "Parents are worried that their kids might be left behind," he tells her. "Especially the third-graders."

Such is the depth to which a climate of accountability through high-stakes testing has permeated the education landscape - and a small example of the anxiety it is provoking. Setting the tone nationwide is No Child Left Behind, the sweeping 2002 education reform act, with its strict annual testing requirements and its goal to elevate all students to grade level in reading and math by 2014.

But here in New York City, the debate over the best way to stamp out social promotion - the practice of graduating students with their peers, regardless of whether or not they are prepared academically - has taken on an urgency perhaps not currently matched anywhere else in the United States.

Beginning this year, a new policy will take effect in all of the city's public schools. Third-graders who cannot demonstrate basic competency on citywide math and English tests will not be promoted to the fourth grade.

For those who work in schools that function at a high level, this may not seem such a radical concept. In New York, however - the nation's largest school system with over 1 million students - firm enforcement of the new rule may mean requiring thousands of third-graders to repeat a year surrounded by younger students.

With all eyes watching to see if his campaign proves visionary or hopelessly quixotic, Mayor Michael Bloomberg is forging ahead to toughen promotion requirements.

Already, Mr. Bloomberg has expended more political capital than many would think wise. When the majority of his advisory Panel for Educational Policy opposed the move, Bloomberg fired the naysayers and replaced them with supporters - unleashing a howl of protests branding him a dictator.

But what some observers say may be even more perilous is the degree to which his adherence to the tough policy seems strikingly at odds with lessons learned from recent past experience.

A significant body of research indicates that holding students back increases their likelihood of becoming discouraged and dropping out of school.

New York itself tried - and failed - in the 1980s to stiffen promotion requirements.

And even as the debate rages here, a major report about to be released in Chicago is expected to send a mixed message, at best, about that city's experiment with stamping out social promotion.

Many New Yorkers on the front line of this question - principals, teachers, and parents who work every day beside the city's students - also remain deeply skeptical.

"With kids as young as third grade, you can't have a policy of one size fits all," says Sheldon Benardo, principal of PS 86, of using standardized test scores to quantify student achievement.

Then, there is the ever-troubling issue of test performance.

Ammon Ford, a senior at Long Island City High School in Queens, knows firsthand that not all students excel at taking tests. His AP scores don't reflect his grades, so how could a standardized exam define a third-grader's ability?

"Don't hold them back because they aren't good at taking tests," he says.

Yet, in defense of Bloomberg's plan, there is also a handful of current studies showing that retention - when infused with extras like individualized learning plans and one-on-one tutoring - may have some benefits.

In general, though, those benefits tend to appear only over a longer period - and are unlikely to produce the kinds of quick results favored by politicians.

It may be a matter of Bloomberg's general approach to education, which Ammon likens to the pragmatic way one might tackle a "business venture."

This is both a criticism and a compliment frequently lobbed at the mayor, a billionaire businessman who appointed Joel Klein, a former federal prosecutor, as his head education official.

But besides placing too much emphasis on hard numbers, the new policy diminishes the role of teacher input, educators worry.

"You can't leave the teacher out of the process," says Laura Reyes, who instructs bilingual third-graders.

Ms. Reyes's concern underscores what many see as a disconnect between policymakers and educators.

"We're with the children 180 days a year," she says. Yet it's the education officials in their Tweed Courthouse headquarters who are making blanket decisions about their individual progress.

Mr. Benardo says that as a principal, even he is too far removed to evaluate students without consulting their teachers.

As New York's policy now stands, third- graders who score in the lowest of four levels on the city's reading or math tests will not move ahead to the fourth grade.

Students may retake the test in August, after attending summer school. As a result of cries from concerned parents, they may also appeal to be moved ahead based on criteria such as class work.

But even with the appeals process tacked on, a legion of third-graders - 15,000, or 20 percent - faces the possibility of repeating third grade.

While she doesn't plan to fail all of them, Deborah Sachs has identified 17 of her 28 third-graders as at risk for being held back. And though she supports retention and feels it's better to remediate students when they're young, she says that if all struggling children were held back, "there would be no fourth grade."

The question policymakers should be asking, says Paul Reville, a lecturer at the Harvard Graduate School of Education is: "How's the second year going to be better than the first for this child?"

Slamming social promotion, after all, is nothing new. What's needed this time around, many argue, is a new definition of what it means to be retained.

"Most things in education are cyclical," notes Kathy Christie of the Education Commission of the States. "And this is one of them for sure."

The pendulum has swung from social promotion and back to retention since the 1930s, when strict standards were the norm. Tough promotion policies surged again in the 1950s and the '80s.

Yet large urban districts such as Baltimore and Chicago offer cautionary tales against simply recycling students through the same grade.

In 1982, Karl Alexander, a sociology professor at Johns Hopkins, embarked on a long-term study of 800 Baltimore first-graders. About half were held back at some point in their academic careers without being offered any extra support.

What he found defied the accepted ideas about retention: Student test scores and grades actually did improve. But they still lagged far behind grade level.

A more recent study by Brian Jacob, a professor of public policy at Harvard's Kennedy School of Government, examined the records of Chicago public school students in the late 1990s and found that retention increased achievement for low-performing third-graders there.

In what became a bellwether for similar efforts, Mayor Richard Daley in 1996 set out to end social promotion in Chicago public schools, much as Bloomberg is doing now in New York. Yet in part because of the findings from a study by the Consortium on Chicago School Research to be released today, that city has reexamined and revised its policy. Math scores will no longer be used to retain students, and no child will repeat the same grade twice.

Chicago is also shifting its focus to individualized assessment in the earlier grades according to Barbara Eason-Watkins, Chicago's chief education officer.

Michele Cahill, senior counselor for education policy in New York, acknowledges that research has shown that "retention alone is not a policy that can be supported." Therefore, in addition to summer school, New York will offer extended days and tutoring over school breaks. In all, $90 million will be pumped into summer school and tutoring this year and next. But critics wonder if it will be enough.

Today, most education experts argue that social promotion and retention are equally flawed approaches. Instead, they say, attention should be paid to the earlier grades. "The sooner you can pick up the problem the better," says Professor Alexander. "Third grade is often too late."

"We do these heroic interventions when things reach a crisis point. The preferred course of action would be to intervene before the problems get that severe."

He says "What we need are some creative third ways." Whether Bloomberg's plan will prove to be such a creative third solution, or simply "retention repackaged," remains to be seen.